Oil sand, as known in the Fort McMurray region of Alberta, Canada, comprises water-wet sand grains having viscous bitumen flecks trapped between the grains. The oil sand lends itself to separating or dispersing the bitumen from the sand grains by slurrying the as-mined oil sand in water so that the bitumen flecks move into the aqueous phase.
For the past 25 years, the bitumen in McMurray oil sand has been commercially recovered using a hot/warm water process. In general, the process involves slurrying oil sand with heated water, optionally, a process aid such as caustic (NaOH) and naturally entrained air. The slurry is mixed, commonly in tumblers, for a prescribed retention time to initiate a preliminary separation or dispersal of the bitumen and the solids and to induce air bubbles to contact and aerate the bitumen. The conditioned slurry is then subjected to flotation to further separate the bitumen from the sand.
A recent development in the recovery of bitumen from oil sand involves a low temperature process whereby the oil sand is mixed with heated water directly at the mine site to produce a pumpable, dense, low temperature slurry. The slurry is then pumped through a pipeline to condition the slurry for flotation. It is understood, however, that other bitumen extraction processes exist, each producing conditioned oil sand slurry.
Conditioned oil sand slurry may be further diluted with flood water and introduced into a large, open-topped, conical-bottomed, cylindrical vessel (termed a primary separation vessel or “PSV”). The diluted slurry is retained in the PSV under quiescent conditions for a prescribed retention period. During this period, the aerated bitumen rises and forms a froth layer, which overflows the top lip of the vessel and is conveyed away in a launder. The sand grains sink and are concentrated in the conical bottom. They leave the bottom of the vessel as a wet tailings stream. Middlings, a watery mixture containing solids and bitumen, extend between the froth and sand layers.
The wet tailings and middlings are withdrawn and may be combined for further processing in a secondary flotation process. This secondary flotation process is commonly carried out in a deep cone vessel wherein air is sparged into the vessel to assist with flotation. This vessel is referred to as the TOR vessel. It and the process conducted in it are disclosed in U.S. Pat. No. 4,545,892, incorporated herein by reference. The bitumen recovered by the TOR vessel is recycled to the PSV. The middlings from the deep cone vessel are further processed in air flotation cells to recover contained bitumen.
The froths produced by these units are generally combined and subjected to further processing. More particularly, it is conventional to dilute the bitumen froth with a light hydrocarbon diluent, such as naphtha or a paraffinic diluent, to first improve the difference in specific gravity between the bitumen and water and to reduce the bitumen viscosity, to aid in the separation of the water and solids from the bitumen. Separation of the bitumen from water and solids is commonly achieved by treating the diluent diluted froth in a sequence of inclined plate settlers, scroll and disc centrifuges, and the like. Other processes for separating solids and water from diluted bitumen froth are known in the art and include stationary froth treatment (SFT) as described in U.S. Pat. No. 6,746,599, incorporated herein by reference.
The primarily water and solids fraction obtained after separation is commonly referred to as froth treatment tailings. Paraffinic froth treatment tailings typically comprise water, asphaltenes, fines solids, bitumen and about 5010 wt % residual paraffinic solvent. Naphthenic froth tailings typically comprise water, fines solids, residual bitumen and about 2-4 wt % naphtha. It is desirable both economically and environmentally to recover the hydrocarbon diluent from the tailings prior to disposal of the tailings. However, the unique nature of the diluent-containing tailings makes diluent removal a challenge to the industry. In particular, it is believed that some of the diluent is intimately associated with the solids, making diluent removal from the solids more difficult.
Canadian Patent No. 1,027,501 discloses a process for treatment of centrifuge tailings to recover naphtha. The process comprises introducing the tailings into a vacuum flash vessel maintained at vacuum conditions (e.g., about 35 kPa) in order to flash the naphtha present in the tailings. The vessel is also equipped with a plurality of shed decks so that any residual naphtha remaining in the tailings stream will be vaporized by the introduction of steam beneath these shed decks. In practice, however, this process results in only 60 to 65% recovery of the diluent, as the vacuum at the tailings feed inlet of the vessel may have resulted in the tailings bypassing the shed decks and pooling near the bottom of the vessel. In the alternative, or additionally, the reduction in pressure in the tower to below atmospheric resulted in steam condensation and reduced heat transfer to the slurry. Thus, the pooled tailings at the bottom of the vessel still contained a substantially large amount of diluent. Canadian Patent No. 2,272,035 partially addressed this issue by introducing the steam into the tailings pool for vaporizing the residual diluent pooling near the bottom of the vessel.
Canadian Patent No. 2,272,045 discloses a method for recovery of hydrocarbon diluent from tailings produced in a bitumen froth treatment plant comprising introducing the tailings into a steam stripping vessel maintained at near atmospheric pressure (e.g. around 95 kPa) in an attempt to avoid the problem of the tailings bypassing the shed decks. Without a vacuum, vessel pressure increased to atmospheric, or slightly above, and temperature increased to around 100° C. This resulted in increased steam to slurry heat transfer, greater steam flowrate to the condenser and consequently increased naphtha recovery. The operating temperature of the vessel was preferably maintained at approximately 100° C.
However, while operating a steam stripping vessel for recovery of hydrocarbon diluent from tailings produced in a bitumen froth treatment plant at about 100° C. and at near atmospheric pressure significantly improved diluent recovery over previous operations at below atmospheric pressure, there still was a substantial amount of diluent remaining in the tailings pool. As stated in Canadian Patent No. 2,272,045, operating the vessel at near atmospheric pressure and at a steam to tailings ratio of approximately 9.0 wt. % increased the naphtha recovery to only about 80%.